I have the original Platinum, and find the connectors at the front very useful, I also use the optical output with my Videologic DigiTheatre.
BCYL:
You have the option to output to a 5.1 decoder, by still using the yellow digital output on the live's card.
With the newer Sb Live 5.1, it can now do internal 5.1 decoding, where the 6 channels are outputted through three jacks in the back, one the these jacks is the yellow digital out.
So basically, the yellow digital out has a dual use.